1. Field of the Invention
The present invention relates generally to the manufacturing of optical fibers, and particularly to manufacturing a fluorine doped preform from which an optical fiber may be drawn from the preform.
2. Technical Background
Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has had a significant impact in the local area networks (i.e., for fiber-to-home uses), which have seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver ever-greater volumes of information in the form of data, audio, and video signals to residential and commercial users. In addition, use of optical fibers in home and commercial business environments for internal data, voice, and video communications has begun and is expected to increase.
Optical fibers having a fluorine doped region have unique attributes which make them useful as long haul optical fibers, dispersion compensating optical fibers, dispersion slope compensating optical fibers, and high data rate optical fibers. The ability to include fluorine in a preform is an important aspect of producing an optical fiber with a fluorine-doped region.
Prior attempts to incorporate fluorine into the preform include depositing fluorine-doped soot on a starting member. Typically, the starting member was a sintered core cane. During deposition, CF4 is usually added to the fume tube of the burner with SiCl4 and O2 and reacted in a CH4/O2 flame. The fume tube or fume is used herein to describe an opening in the center of the exterior surface of the burner. However, in the past, deposited fluorine has shown to be a volatile compound and exhibited significant migration from the region or regions of interest. In addition to migration of fluorine in the preform, the fluorine incorporated into the soot preform during deposition can vaporize and diffuse from the preform either during subsequent deposition or during consolidation. The diffusion of fluorine from the preform and migration of fluorine from regions of interest has collectively been commonly known as “loss”, referring to the loss of deposited fluorine from areas of interest of the sintered preform. Preforms fluorinated during deposition have exhibited a fluorine loss between forty (40%) percent to fifty (50%) weight percent. In light of fluorine migration and diffusion, the deposition of fluorinated soot has not proven to be practical.
The aforementioned loss of fluorine can also cause a loss of profile shape, reduction in depth of a fluorine moat in resultant preform or fiber, and/or induce significant aging effects such as the fiber exhibiting an increase in attenuation as the useful life of the fiber is extended.
One reason for the low retention rate of fluorine is the production of the compound SiF4 during manufacturing. Typically SiF4 generated during manufacturing can volatilize off during subsequent high temperature processing such as additional deposition or consolidation.
A countermeasure to the migration is to confine the fluorine containing region in the soot blank by bounding the area to have a fluorine-doped region between two densified glass barrier layers. However, methods of forming the barrier layers can be significant sources of attenuation of a signal traveling down the fiber. Known methods of forming a barrier layer include heating at least a layer of uncondsolidated soot to sintered glass with an oxy-hydroylsis torch and to deposit soot on the sintered glass layer. The glass surface formed by the torch heating step can contain a significant amount of hydroxyl ions, which are known attenuation sources.
With respect to consolidation doping, CF4 is typically added to the consolidation atmosphere after a Cl2 drying step. The fluorine content of the soot preform reaches an equilibrium with the gas phase CF4 and the sintering of the preform traps a certain level of fluorine in the sintered glass. Consolidation doping may be used to achieve a fluorine level of up to 2.1 wt % (−0.6 Δ% compared to undoped silica) of fluorine in the sintered glass. A disadvantage of consolidation doping is that every region in the profile containing or bordering a fluorine-doped region will require a separate deposition-consolidation cycle. This greatly increases the process steps for making the glass preform and reduces potential capacity of the glass manufacturing facility.
A fluorine doping method is needed that enables deeper moat levels which does not reduce deposition rates to impractical levels.